Apatheia in the Christian Tradition

To many modern people, apatheia (being "without suffering"/"without passion") sounds like cold-heartedness and indifference to others, a condition to be avoided. However, in the classical world and for many in the historic Christian church it was a spiritual state to aspire to. What exactly is apatheia? What is its origin? How has it been used in spiritual writings throughout the centuries of Chri...